TeleTYpewriters or TTYs are typewriter-style devices for communicating alphanumeric information over telecommunication networks. TTYs are equipped with modems, which accept digital data, and modulate it into a form suitable for transmission over the PSTN such that a corresponding receiving modem can demodulate the signal back into the original digital data.
TTYs have found particular utility in enabling the hearing and speech impaired to communicate over the Public Switched Telephone Network or PSTN. This type of TTY is referred to as a TTY/Telecommunication Device for the Deaf or TDD. The TTY transmission protocol most commonly used in the United States (45.45 baud Frequency Shift Keying or FSK Baudot signaling) has no carrier tone. For this reason, a Baudot TTY call can be put on hold or transferred to another party without causing the automatic disconnect that would ordinarily occur with devices requiring a continuous carrier tone. A drawback of the 45.45 baud Baudot protocol is that the maximum transmission speed is approximately six characters per second, which is considerably slower than most people are able to type.
Due to this problem, the International Telecommunications Union (“ITU”) (formerly known as the CCITT) has set various modulation standards for modems that allow much faster data transmission rates. Examples of ITU standards are V.21 (used in the European Union), V.22, V.22 Bis, V.32, V.32 Bis, and V.34. In the United States, prior to the adoption of ITU standards, various “Bell standards” were established. Examples of Bell standards are Bell 103, Bell 202, and Bell 212. As used herein, an “ITU modem” is a modem that may operate using an ITU modulation/demodulation protocol (or standard). Typically, ITU modems may also operate in at least one Bell modulation/demodulation protocol. A “Bell modem” may operate in a Bell modulation/demodulation protocol. Most modems currently being sold in the United States are ITU modems that are capable of transmitting and receiving in various ITU modulation/demodulation protocols as well as Bell modulation/demodulation protocols. The term “ASCII modem” is sometimes used to refer to an ITU or Bell modem.
Despite the significant speed advancement of these protocols relative to the Baudot protocol, calls using the ITU protocols cannot be put on hold or transferred without disconnecting the call. TTYs using the protocols have a “carrier tone” which is a constant audible signal. Information is exchanged by varying the amplitude and/or frequency of the carrier tone. If this signal is lost because the call is placed on hold or transferred, the other TTY will hang up and not reestablish the connection with a different modem. Modems are very inflexible in their protocol requirements and will not operate properly or will abort the call if the protocol settings are wrong. Because it is not possible to transfer a call from one physical workstation to another, many contact centers servicing the hearing and speech impaired require agents to physically switch seats, while the call is in progress, to have a new agent service the call. In addition to being inconvenient, this approach prevents the call center from load balancing among multiple call centers that are part of the same enterprise. The use of carrier tones poses significant obstacles to the operation and effectiveness of Section 508-sensitive government agencies, call centers, medical facilities, and Public Safety Access Points or PSAPs. Because of the carrier tone, the TTY calls cannot be put on hold or serviced by an automated attendant application to route the call within the enterprise network.
This problem has been largely overcome in Voice over IP or VoIP systems by requiring the PSTN gateway (rather than an actual TTY device behind the gateway) to maintain the carrier tone-based connection with the analog device on the PSTN. Within the VoIP network itself, the corresponding communication between the gateway and the VoIP terminal is in the form of text, rather than modem signals. The text stream between the VoIP terminal and the system's PSTN gateway can be transferred to a different VoIP terminal without causing an interruption of the carrier tone between the gateway and the PSTN device.
The Time Division Multiplexed (TDM) architecture that most closely resembles the above-described VoIP technique is the “modem pool” approach that was first implemented commercially a number of years ago. In a typical configuration, a bank of modems, either internal or external to the Private Branch eXchange or PBX, provides a means of exchanging analog-encoded data or facsimile images with interfaces or systems on the trunk side of the PBX. As is true of the VoIP mechanism, analog interfaces and carrier tone support exist only on the outward-facing trunk connection. Within the PBX environment, data are conveyed to and from stations in one of a variety of digitally encoded protocols, rather than by the analog, tones-based protocol used for the external interface. For example, for past AT&T, Lucent, and Avaya systems, the internal PBX protocol used for communicating with a member of the modem pool was Digital Communications Protocol Mode 2 (a synchronous digital interface) or DCP Mode 3 (an asynchronous equivalent).
A deficiency of the VoIP and TDM architectures of the prior art is that problems can arise when analog modem-based devices, such as non-Baudot TTYs, are used within PBX environments. In this configuration, it is important to note that the trunk-facing modems associated with the PBX are not employed. As a result, when modem-based devices within the PBX environment communicate directly with similar devices on the PSTN, calls cannot be transferred or put on hold.
There is a need for a system that permits calls involving modem-based devices within the PBX environment to be transferred and put on hold.